Thiolates vs. halides as pi-donors: the redox-active alkyne complexes [M(SR)L(eta-R'C[triple bond]CR')L'] {M = Mo or W, L = CO or P(OMe)(3), L' = eta-C(5)H(5) and Tp'}

The cyclic voltammograms of the alkyne complexes [M(SR)L(eta-R'C[triple bond, length as m-dash]CR')(eta-C(5)H(5))] (M = Mo or W, R = Me or Ph, R' = Me or Ph) show two oxidation processes. Both are irreversible for the stereochemically rigid carbonyls (L = CO) but the first is reversible for the fluxional phosphites {L = P(OMe)(3)}; the paramagnetic monocations [M(SPh){P(OMe)(3)}(eta-MeC[triple bond, length as m-dash]CMe)(eta-C(5)H(5))](+) were detected by ESR spectroscopy after in situ chemical one-electron oxidation. By contrast, the hydrotris(pyrazolyl)borate analogues [W(SR)(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'] {R = Me or Ph, Tp' = hydrotris(3,5-dimethylpyrazolyl)borate} are oxidised in two reversible steps to the corresponding mono- and dications; the redox pair [W(SPh)(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'](z) (z = 0 and 1+) has been structurally characterised. A comparison of the redox potentials for the oxidation of [W(SR)(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'] with those of the halide analogues [WX(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'] suggests that the factors which give rise to the inverse halide order for the latter may not operate for the thiolates, which appear to be the better pi-donors in all three redox states [WL(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'](z) (L = halide or thiolate, z = 0, 1+ and 2+).